The present invention relates to a nonvolatile memory which permits electrical writing and erasion of information to be stored, and to a technique that can be effectively applied to a flash memory provided with a replacing function to any defective area in memory array and having an area in which to store a management information table for managing defective addresses.
A flash memory uses as its memory cells nonvolatile memory elements each consisting of a double gate-structured MOSFET having a control gate and a floating gate, and is caused to store information by varying the accumulated charge of the floating gate and thereby varying the threshold voltage of the MOSFET.
In a flash memory, the threshold voltage is varied when data are to be written into memory cells or data therein are to be erased, the currently available manufacturing techniques cannot avoid uneven variations of the threshold voltage due to uneven characteristics of memory cells even if writing or erasion is done under the same conditions, and sometimes there arise a defective memory cell or cells which do not allow sufficient threshold voltage variation.
A conventional flash memory is often provided with a replacing function to replace a prescribed memory area, when there arises any defective memory cell not allowing the threshold voltage to vary sufficiently, involving that defective memory cell with another normal memory area and another area in which to store management table information for managing defective addresses.